The invention relates generally to hydrostatically supported rolls and, more particularly, to an improved sealing element for such a roll.
A roll having piston-like sealing elements of this type is disclosed in DE-OS 3640903, which is a foreign counterpart to the assignee's allowed U.S. application Ser. No. 07/126,773, filed Nov. 30, 1987, now U.S. Pat. No. 4,856,157, entitled Roll With Controllable Line Pressure. Sealing elements of this type can function both as "overpressure" and "underpressure" elements. What this means is that the clearance space between the crosshead and the inner circumference of the hollow cylinder of the roll, outside of the sealing elements can either be empty or filled with hydraulic fluid at a predetermined pressure. When the force exerted against the inner circumference of the hollow cylinder by the hydraulic fluid in the pressure chamber of a sealing element is greater than the force that would have been exerted against the corresponding sectional area of the inner circumference of the hollow cylinder if no sealing element were present, the sealing element functions as an "overpressure" element. Therefore, a pressure that is greater than the pressure existing in the clearance space is exerted locally, by the effective sectional area of the sealing element, against the inner circumference of the hollow cylinder, and the sealing element functions in the same way as the supporting elements disclosed in DE-AS 2230139. When the sealing element functions as an "underpressure" element, the clearance space between the crosshead and the inner circumference of the hollow cylinder outside of the sealing element is filled with hydraulic fluid at a predetermined pressure and the pressure in the area of the sealing element is lower than the ambient pressure in the clearance space. In this manner, the sealing element causes a reduced pressure zone or a "hole" to be formed in the surrounding volume of hydraulic fluid in the clearance space. Thus, either low pressure or no pressure at all prevails in this zone or "hole" when the sealing element functions as an "underpressure" element. Such a sealing element function is described in DE-OS 3640902, which also is a counterpart foreign application to allowed U.S application Ser. No 07/126,773 discussed above.
When the sealing elements are to operate as "overpressure" elements in accordance with DE-AS 2230139 and accordingly, a high pressure prevails in the pressure chamber, the pressure chambers of the sealing elements, including the choke bores, must be sealed such that the required contact pressure can develop at the inner circumference of the hollow cylinder. However, when the sealing elements are to operate as low pressure elements, the hydraulic fluid entering into the pressure chamber must be able to flow out unhindered, i.e., without a significant pressure build up. Thus, the resistance to flow of the sealing element must be different in each of the two flow directions. For this purpose, nonreturn or check valves, which are closed in the direction of flow into the pressure chambers and open in the reverse direction, are mounted in the sealing elements of the DE-AS 3640902. As discussed above, the fundamental design of the sealing elements in high pressure mode of operation corresponds generally to that of the supporting elements of DE-AS 2230139.
Use of these nonreturn valves entails a certain expense, which makes the sealing elements more expensive. Moreover, their functioning can be adversely affected by impurities, which are carried in the hydraulic fluid and settle onto the valve seat. In addition, when a known sealing element functions as an "overpressure" element and one or more of its choke bores, which lead to one or more pressure chambers adjacent the inner circumference of the cylinder, becomes clogged, a sufficient amount of hydraulic fluid no longer can flow into its respective pressure chamber and then over its rim. This condition endangers the formation of a load-bearing liquid film between the sealing element and hollow cylinder. The resulting metallic friction and corrosion, which can appear on the inner circumference of the hollow cylinder, may cause the respective sealing element to breakdown and, in an extreme case, can even prevent rotation of the roll.